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Old 21 Aug 2002, 09:30 (Ref:362746)   #1
madmat666
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few Qs about brake discs

im doing a physics report on brake discs and have a few unanswered questions:
racing discs need get up to operating temp to be the most efficient (400-600 degrees i think it is for F1) why is this??

and brake fade what exactley is this and why does it happen? (i dont drive my self so start from scratch on that one

thanks in advance

matt
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Old 21 Aug 2002, 10:21 (Ref:362790)   #2
Rhino
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It's all about friction, the more friction you have between pad and disc the more you turn forward speed into heat. In motorsport you want to stop as quick as possible. Therefore you will need higher friction, which creates more heat than a road car. The pad and disc material will have to be able to face higher temperatures. Consequently the starting temperature is higher and their no good for driving round town!
Carbon discs are more favoured by race teams because their lighter(unsprung weight on suspension) last longer and have a slightly higher operating temperature than steel brakes.
Brake fade is one of two things
1/ Brakes get too hot and brake fluid boils, meaning less pad pressure on the disc, and loss of braking efficiency, and spongy pedal.
2/ The pad gets too hot and the material glazes meaning less friction and loss of braking efficiency.
I hope these will help.
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Old 21 Aug 2002, 10:52 (Ref:362818)   #3
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i think the operating temp of carbon discs is a lot higher.. more like 800c?

brake fad also happens when the pads wear out completely lol.
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Old 21 Aug 2002, 11:32 (Ref:362851)   #4
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""One of the most astonishing parts of an F1 car are the brakes. They can decelerate the car from 200 mph to 50 mph in only 3 seconds, taking 100 metres. The forces involved in slowing the car are phenomenal, with 4G often created under heavy braking.


F1 cars use disc brakes like most road cars, but these brakes are designed to work at 750 degrees C and are discarded after each race.

For qualifying, when longevity of the brake discs is not important, teams often run thinner discs to reduce the 'unsprung' weight of the car. Race discs are 28 mm thick (the maximum allowed) where the special qualifying discs are often as thin as 21 mm. Ferrari often uses lightweight calipers during qualifying as well. Teams often run either very small or in some cases no front brake ducts during qualifying to gain an aerodynamic advantage.

BRAKE MANUFACTURE

These brakes are extremely expensive as they are made from hi-tech carbon materials (long chain carbon, as in carbon fibre) and they can take up to 5 months to produce a single brake disk. The first stage in making a disc is to heat white polyacrylo nitrile (PAN) fibres until they turn black. This makes them pre-oxidised, and are arranged in layers similar to felt. They are then cut into shape and carbonised to obtain very pure carbon fibres. Next, they undergo two densification heat cycles at around 1000 degrees Celsius. These stages last hundreds of hours, during which a hydrocarbon-rich gas in injected into the oven or furnace. This helps the layers of felt-like material to fuse together and form a solid material. The finished disc is then machined to size ready for installing onto the car.

There are two main companies that make brakes for F1 cars, AP Racing and Brembo (Brembo has recently acquired AP Racing). Carbon discs and pads are more abrasive than steel and dissipate heat better making them advantageous. Steel brakes as used in CART are heavier and have disadvantages in distortion and heat transfer. Metal brake discs weigh about 3 Kg, carbon systems typically 1.4 Kg. Metal brakes are advantageous in some aspects such as 'feel'. The driver can get more feedback from metal brakes than carbon brakes, with the carbon systems often being described like an on-off switch. The coefficient of friction between the pads and the discs can be as much as 0.6 when the brakes are up to temperature. You can often see the brake discs glowing during a race, this is due to the high temperatures in the disc, with the normal operating temperature between 400-800 degrees Celsius.""

Hope this helps
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Old 22 Aug 2002, 10:39 (Ref:363533)   #5
madmat666
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thanks a million for this, very helpfull
about coefficient of friction how is this determined?
i know its a materials constant and that F = mN
F Friction
m coefficient of friction
N normal force
can that equation be applied to brake discs?
at what sort of temperatures does the brake fluid boil? do they have special fluids for this reason?
matt

Last edited by madmat666; 22 Aug 2002 at 10:43.
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Old 22 Aug 2002, 11:10 (Ref:363560)   #6
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Quote:
Originally posted by madmat666
thanks a million for this, very helpfull
about coefficient of friction how is this determined?
i know its a materials constant and that F = mN
F Friction
m coefficient of friction
N normal force
can that equation be applied to brake discs?
at what sort of temperatures does the brake fluid boil? do they have special fluids for this reason?
matt
Friction coefficient is determined experimentally; there is a standard test, specified by the SAE, for this, which uses a steel plate.

The SAE also specifies friction codes, which you will see printed on pads, as two letters. The codes are:

C <.15
D .15 to .25
E .25 to .35
F .35 to .45
G .45 to .55
H .55 & higher.

The two letters on pads indicate cold & hot friction; most road car pads are FF, sintered motorcycle pads are HH.

Friction coefficient is also dependent on disc material - cast iron will give something like 20% higher levels than steel.

You can apply that equation to brake discs. That will give you a braking force; multiply that force by the effective radius (distance from wheel centre to centre of area of the pad) & you get a torque; add in the speed of rotation of the disc & you you'll get braking power - you can now use your basic dynamics formulae to calculate stopping distances, etc.

You'll note that the basic friction equation does not make any reference to area! In theory, a very tiny pad would do just as good a job as a very big one.....reality dictates otherwise. Two important factors are the shear strength of the pad material & the amount of energy, in the form of heat, which the pad is subjected to.
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Old 22 Aug 2002, 18:22 (Ref:363824)   #7
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Traditionally brake fade was a common characteristic of drum brakes where the friction pads, or shoes, are fully enclosed by a drum which the shoes expand outwards against. When the drum heats up with friction it expands away from the brake shoes thus reducing brake performance. Brake fade shouldn't really happen on disc brakes under normal circumstances. One of the reasons for discs being universally adopted was to get away from brake fade.
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Old 22 Aug 2002, 18:52 (Ref:363835)   #8
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"Brake fade shouldn't really happen on disc brakes under normal circumstances"

I'm not so sure JGM. Even on a lightweight, relatively low powered Formula Ford (with disc brakes), brake fade was a problem on short courses, or at the end of a long series of hard brake turns.

For me though, brain fade was more frequently the problem.
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Old 23 Aug 2002, 04:09 (Ref:364170)   #9
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I tend to agree with you Neil C.
I use a small sedan for club racing (sprints/ hillclimbs).
It has recieved quite a brake upgrade, that includes 4 pot calipers up front and two pots at rear. At a recent sprint, tried out a new pad compound to try and get some cold temp feel into the brakes. Due to the tight twisty nature of the circuit, within 1 and half laps, you got that putrid brake smell and it didn't want to slow up for the corners. Went back to the hotter pads and not a problem.
Brake fade does exist, even with 4 wheel disc on a sedan weighing 900kgs.
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Old 23 Aug 2002, 10:23 (Ref:364309)   #10
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Disc brakes don't fade anything like as much as drums, but fade can still be a problem. The basic cause of fade is overheating of the friction material, which causes a drastic reduction in friction level. Gases & other byproducts are given off - allowing these to escape by grooving the disc or the pad helps reduce fade.

The formulation of organic friction materials is basically a trade-off between friction coefficient & fade resistance. A 'soft' pad will give very good cold friction, but hot friction will be poor, while a 'hard' pad has the opposite characteristics.
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Old 24 Aug 2002, 06:56 (Ref:364879)   #11
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They fade!
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